1. Field of the Invention
The present invention relates to a power supply circuit used in a liquid crystal display (LCD) apparatus, for example.
2. Description of the Related Art
Generally, in an LCD apparatus including an LCD panel, a data line (signal line) driver circuit and a gate line (scan line) driver circuit, a power supply circuit is provided to supply regulated voltages to the data line driver circuit and the gate line driver circuit.
In the data line driver circuit, the regulated voltage supplied thereto does not need to be high; however, a high current drive ability is required. Therefore, the regulated voltage is derived from 2·VDD, for example, where VDD is the power supply voltage. On the other hand, in the gate line driver circuit, the regulated voltages supplied thereto do not require a high current drive ability; however, the regulated voltages are a sufficiently high voltage and a sufficiently low voltage. Therefore, the regulated voltages are derived from 4·VDD and (−2)·VDD, for example. This will be explained later in detail.
A prior art power supply circuit is constructed by a step-up circuit for receiving a power supply voltage VDD to generate a 4-multiple step-up voltage 4·VDD, a 2-multiple step-up voltage 2·VDD and a (−2)-multiple step-up voltage (−2)·VDD, a first voltage regulator powered by the 4-multiple step-up voltage and the ground voltage to generate a first regulated voltage, a second voltage regulator powered by the 2-multiple step-up voltage and the ground voltage to generate a second regulated voltage, and a third voltage regulator powered by the ground voltage and the (−2)-multiple step-up voltage. This also will be explained later in detail.
In the above-described prior art power supply circuit, however, the difference between the two power voltages of the first voltage regulator is 4·VDD, the difference between the two power voltages of the second voltage regulator is 2·VDD, and the difference between the two power voltages of the third voltage regulator is 2·VDD. As a result, all of the first, second and third voltage regulators are not always stably operated, or the reliability of some of the voltage regulators may deteriorate.